There you go - you've worked out that TC fields will do what you wanted to achieve.

To get TC fields to show in a TOC at different levels, you simply include the switch and level in the field.
{TC "This is text that will be TOC 1" \l 1}
{TC "TOC 3 is the style that this one will use in the TOC" \l 3}

As you have discovered, these TC fields need to be in the body of the document (not the footnote area), positioned immediately after the Footnote Reference that they apply to.

Then your Table of Contents field which interleaves headings with the TC fields would include switches for both
{TOC \t "Heading 1,1Heading 2,2" \f \h}
or
{TOC \o "1-4" \f \h}

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Andrew Lockton
Chrysalis Design, Melbourne Australia